Recent efforts have continued in characterization of the biological and molecular features of Mycoplasma genitalium, in anticipation that this information will lead to development of improved methods in detection of the organism and more definite confirmation of its role in human disease. Mycoplasma genitalium, first isolated from the urethra of two men with nongonococcal urethritis in our laboratory in 1980, has now been found retrospectively in throat specimens of military recruits participating in an inactivated M. pneumoniae vaccine field trial in 1974-75. Three of sixteen preserved throat isolates, previously identified as M. pneumoniae, have now been shown to be mixtures of M. pneumoniae and M. genitalium. Purification of these mixed mycoplasms by selection of single colony isolates clearly confirmed the presence of M. genitalium. Identification of M. genitalium was based upon the occurrence of species-specific 140 kDa protein adhesin in these isolates and their extensive serologic reactivity to a M. genitalium antiserum. The frequent occurrence of both M. genitalium and M. pneumoniae in a number of these throat specimens, in combination with shared serologic reactions between these two mycoplasms, suggests the likelihood that M. genitalium strains might be easily missed in current diagnostic laboratory techniques. What role M. genitalium might have in human respiratory disease remains to determined. Further studies on other mollicutes, primarily of insect orgin, have led to discription of new species of Acholeplasma, the isolation of acholeplasmas from human amniotic fluid, revised reclassification of helical mollicutes (spiroplasms), and an enlarging group of other mollicutes that have been difficult to assign to established taxonomic groups.